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Herr Zapp

Tue May 26 2009, 09:55PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

Sorry, for some reason I did not see your posting with the spark gap details.

Although this gap design will "work" (and has worked for many coilers), I don't like it because there is very little airflow through the actual gaps. Even if you fabricate some sort of "plug" that closes up the center of the assembly, the resulting airflow is still parallel to the gaps, rather then perpendicular.

Also, it will be harder than you think to accurately locate the mounting holes for the pipe segments, and get them adjusted properly.

My preference is to make a long, narrow 5-sided box (top open) out of plywood or plastic, and mount two notched "support strips" on the inside of the open top, along the "long sides" of the box. The pipe segments just drop into the notches, and the gap between segments is controlled by the spacing of the notches.

The cooling fan is mounted on one end of the box, blowing air through a large hole. With this design, 100% of the airflow from the fan goes directly through the gaps. Also, the gap segments can simply be lifted out of the box when they become oxidized and need cleaning.

The only "difficulty" is determining the spacing and the depth of the notches in the support strips. If you have a table saw or radial arm saw, this is easy to find experimentally. If you just have a hack saw and some files, its slightly more difficult, but still easily done. Of course if time is really getting short, you can set the gap spacing with appropriate shims and just glue the pipe segments in place using epoxy adhesive. Not elegant, but if you have a high-flowrate fan you may not have any overheating problems with only a 30ma power supply.

The problem with using a conventional "alligator clip" for a primary tap is not only the danger of shorting to adjacent primary turns, but the fact the most clips are "toothed", and actually just make a few tiny spots of "point contact" with the primary conductor. Remember that your peak primary current may approach 100 amps, and a very low resistance connection is required. Besides, most alligator clips are just cheap copper-plated steel.

Regards,
Herr Zapp

J. Aaron Holmes

Tue May 26 2009, 11:22PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Herr Zapp wrote ...

Although this gap design will "work" (and has worked for many coilers), I don't like it because there is very little airflow through the actual gaps. Even if you fabricate some sort of "plug" that closes up the center of the assembly, the resulting airflow is still parallel to the gaps, rather then perpendicular.

Agreed. This is called a "RQ" (Richard Quick) gap, after its creator. In this design, the air flow affects electrode cooling moreso than quenching. Well, the quench is aided somewhat by the cooling... I've never quite understood the popularity of this design. Beyond the "looks cool" factor, it doesn't seem like much of a performer.

One problem with blasting air along the sides of pipe sections is that the air pressure will be relatively low near the ends, causing most of the firing to occur there. This, in turn, will greatly increase the rate at which gap performance degrades. For this reason, I've often wanted to try using capsule-shaped electrodes (e.g., pipe sections with half-spheres on the ends) to discourage end-firing. As it is, I've mostly used the conventional "leaf-blower" gap design, where brass pipe fittings are pointed directly at each other and air is blasted through. I put a few of these assemblies in series, and it seems to work very well.

Cheers,
Aaron, N7OE

Herr Zapp

Wed May 27 2009, 01:21AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Aaron -

I didn't label the circular, fan-cooled, parallel-pipe gap an "R.Q.gap" because it's not clear who actually "invented" it. It may be more accurate to say that R. Quick "popularized" this type of gap. This gap design was also described long ago by the Tesla Coil Builders of Richmond (TCBOR), and similar gaps were used even earlier.

I'm not sure why you think the air pressure in a "pressurized box gap" would be lower near the ends of the pipe segments. Technically, the static pressure would be higher near the ends, where the air flowrate would be lower. However, the sharp edges at the ends of the tubes can (in theory) breakdown at a lower voltage than the central, smooth, full radius section of the pipes.

I have built several of these gaps, and run them with up to 15/120 NST power supplies, and the arcing always took place uniformly along the length of the pipe segments, but never at the ends (no "end-firing"). The pipe segments were lying in notched supports of 3/8" thick polycarbonate or LE phenolic. I've found that if you cut the pipe with a VERY DULL wheel-type pipe cutter, the copper will be necked-down slightly (reduced in diameter) before the wheel finally begins to cut. This reduced-diameter at the ends of the pipes undoubtedly helps prevent arcing at the pipe ends. I also filed a 45 degree chamfer on the cut ends, which further prevents any end-firing. If you cut the pipe segments with a hacksaw, you won't get this "necked-down" feature.

Regards,
Herr Zapp

Blitzorn

Wed May 27 2009, 04:01AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Herr Zapp-

I'm fairly certain aaron was referring to my style, the "R.Q." style, the entire time.

Your concerns are duly noted, how about this for an alternative design...

Let's hear some feedback :)

J. Aaron Holmes

Wed May 27 2009, 04:52AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Herr Zapp wrote ...

I didn't label the circular, fan-cooled, parallel-pipe gap an "R.Q.gap" because it's not clear who actually "invented" it. It may be more accurate to say that R. Quick "popularized" this type of gap. This gap design was also described long ago by the Tesla Coil Builders of Richmond (TCBOR), and similar gaps were used even earlier.

That's fair. I suppose we should all stop using the term 'Terry Filter,' too

Well, I'm not trying to pay R.Q. any undue compliments anyway; I'm dubious of that particular design.

Herr Zapp wrote ...

I'm not sure why you think the air pressure in a "pressurized box gap" would be lower near the ends of the pipe segments. Technically, the static pressure would be higher near the ends, where the air flowrate would be lower. However, the sharp edges at the ends of the tubes can (in theory) breakdown at a lower voltage than the central, smooth, full radius section of the pipes.

Sorry, Herr Zapp, I was referring more to problems I've experienced with parallel pipe gap construction myself. I had a hard time visualizing the arrangement as you described it. That's one of those things where a picture is worth a kiloWord

In the arrangements I was playing with long ago, the pipes were suspended in parallel over a single large airflow opening. with these, the flow was greatly enhanced around the ends. Sparks occurred primarily around the ends of the pipes, despite that a pipe cutter was used and had resulted in slightly rounded ends. :-/ But I shouldn't have generalized. Sorry for the confusion.

Anyway! Not to hijack the thread or anything!

Blitzorn wrote ...

Your concerns are duly noted, how about this for an alternative design...

I'm not sure you'll get enough airflow this way, but I could be wrong. The alignment of all the different air holes seems overcomplicated to me. Perhaps Herr Zapp will post of pic of what he's talking about that will make it clear (I'd love to understand better).

Cheers,
Aaron, N7OE

EDIT: Lastly, here's a pic of my first air-blast gap. Worked well. In fact, it was the one used for the 1kVA run that's in my avatar image

The large "T" connector in the middle was chosen because it fit perfectly over the hose from my ShopVac, which was connected to the exhaust port on the ShopVac (hence it's a "blast" type, not a "sucker" type).

Herr Zapp

Wed May 27 2009, 05:47AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

Yes, Aaron was referring to the "R.Q." gap design.

Your modified design looks interesting, but is probably more complicated than is necessary. Eliminate the funnel and the central slotted PVC pipe, and just attach flat plates to both ends of the large diameter PVC housing. One end plate will have a hole or fitting for the hose from the shop vac, the other plate will be solid.

The "slots" in the housing will be difficult to make unless you have access to a milling machine, so you can just use a line of closely-spaced 3/8" diameter holes, aligned with the gap between each pair of pipes.

I made a gap similar to this long ago, but instead of using a shop vac to provide airflow I just mounted a high-flowrate "muffin" style AC fan directly on one of the end plates. This gap ran cool, quenched well, and generally performed well, but it was difficult to access the gaps for periodic cleaning, which it required after every 15-20 minutes of operation when used with a 15/120 NST power supply.

Aaron -

I'll see if I can find a photo of the old "box gap", of at least throw together a quick sketch. The "end" electrodes were lengths of solid brass rod the same diameter (3/4") as the copper pipe segments, with a tapped hole at one end to attach the terminal on the heavy cable to. Because the pipes & brass rods were just "laying" in slots, they could shifted to different positions to select any number of "active" gaps.

As for the "Terry Filter", it SHOULD be referred to by that name (or perhaps the "Fritz Filter") because it WAS Terry Fritz who first really studied (in a controlled, analytical fashon) the voltage spikes and resonances present across the terminals of a NST driving a Tesla coil tank circuit. After evaluating L-C, R-C, and R-L filter arrays, he refined the R-C filter into the "Terry Filter" of today.

These ground-breaking white papers can be viewed at any of the mirror sites containing all of Terry Fritz' information from his extinct Hot-Streamer.com website. One mirror is at Bart Anderson's Classictesla.com site.

A direct link to Terry's papers:

Regards,
Herr Zapp

Blitzorn

Wed May 27 2009, 05:56AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

J. Aaron Holmes wrote ...

I had a hard time visualizing the arrangement as you described it. That's one of those things where a picture is worth a kiloWord

I couldn't have said it better. Diagram would rock my world.

And aaron, i can probably make it a lot simpler than i drew it, i'm just curious about how the concept looks... i'm fairly confident in the overall premise but i need to fiigure out how to move enough air and get it to pass the right places. I'll give it a shot and fill you in, worst comes to worst i have an "R.Q. (but not really)" and it's not the best it could be but it'll stand up to my school demo and then i'll build a better one. I like your design, but i'm kind if at a point where i just need to build what i have the time and materials for and get a passable (if not optimal) result.

J. Aaron Holmes

Wed May 27 2009, 06:05AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Herr Zapp wrote ...

As for the "Terry Filter", it SHOULD be referred to by that name (or perhaps the "Fritz Filter") because it WAS Terry Fritz who first really studied (in a controlled, analytical fashon)

Understood and duly appreciated, hence the <wink>. I'm just pointing out that, as with the R.Q. gap, we often name things after those who make them popular (with or without the data to back things up), not necessarily the inventors. I think we can all agree that Fritz didn't invent RC filters, and the MOV portion (arguably the most distinguising characteristic of a Terry Filter) was not studied in his papers, IIRC. That was "added to taste" later.

Coronafix

Wed May 27 2009, 06:55AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

I don't think you need to worry too much about forced air quenching, it does help a bit but you'll find that with 5-7 copper pipes as a static spark gap, it will work just fine with your transformer. I used 5 pipes in an adjustable setup, I then built a sucker gap and got a few more inches from that but the vacuum cleaner drove me mad.

Herr Zapp

Wed May 27 2009, 05:47PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn, Coronafix -

I also agree that having a shrieking 2HP, 10K RPM vacuum motor anywhere near a small Tesla coil would be very distracting. The first time you fire a small coil, if it's in the daytime, you will HEAR the crackling of the streamers before you actually see them, especially if the coil is not optimally tuned and the streamers are "thin". The quiet whirring of a small "muffin" fan is far less obtrusive.

I think the crackling/popping/zapping sound of the streamers is an absolutely essential part of the Tesla coil "experience".

Also, you need to be aware of possible voltage drop problems if you run both the coil and a large shop vac from a single extension cord, especially if it is long and/or small gage wire. Large shop vacs can draw 10-12 amps or more, and your AC line voltage can experience a considerable "sag" out at the end of a cheap 50', #18AWG extension cord.

Your NST by itself will draw only a few amps, but add that to 12A from the shop vac with the voltage drop in a long extension cord you may only have 95 VAC at the NST input terminals.

So, if you need to use an extension cord for the coil, use the shortest & largest wire gage cord you have, and if possible power the shop vac from a separate branch circuit.

Regards,
Herr Zapp

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